The invention relates in general to an applying method for an adhesive, and more particularly to an applying method for an adhesive used in micro-fabrication.
Generally speaking, a layer of adhesive, such as hexamethyldisilazane (HMDS), needs to be applied before applying a photoresist on a wafer with a SiO2 layer formed on the entire wafer surface so that the photoresist can adhere to the SiO2 layer in a process of photolithography used in micro-fabrication.
With reference to FIG. 1, a diagram of an apparatus used for applying an adhesive to a SiO2 layer of a wafer is shown. A wafer 100 is placed in a closed container 110 with a supply vent 120, which is connected with a transmission pipeline 130 for conveying an adhesive to the closed container 110. The closed container 110 also includes an exhaust vent or outlet 125, and the exhaust vent 125 is externally connected to a first exhaust pipe 140 and a second exhaust pipe 150. The first exhaust pipe 140 is externally connected to a pump, which can exhaust gas from the closed container 110. The second exhaust pipe 150 is externally connected to a general air-extracting or gas-extracting common pipe, which utilizes the Bernoulli Principle to eliminate the gas in the closed container 110. By using the above-mentioned pump to exhaust the gas from the first exhaust pipe 140 the flow rate is greater than if only the Bernoulli Principle method is used to exhaust the gas from the second exhaust pipe 150.
Referring to FIG. 1 and FIG. 2 simultaneously, FIG. 2 shows a conventional method for applying an adhesive on a SiO2 layer of a wafer. The method includes the following steps. First, in step 210, the coating process of applying an adhesive such as HMDS on an oxidation layer of the wafer 100 is begun. In step 220, gas is exhausted from the first exhaust pipe 140 so as to eliminate a part of the gas in the closed container 110. The operation time of this step is approximately five seconds. Next, in step 230, the gas continues to be exhausted from the first exhaust pipe 140 so as to cause bubbling in the adhesive in the transmission pipeline 130 and convey the bubbled adhesive to the supply vent 120. Operation time of this step is approximately five seconds. In step 240, the gas continues to be exhausted from the second exhaust pipe 150 and ceases to be exhausted from the first exhaust pipe 140. Thus the adhesive reaches a gasification state, and the gasified adhesive is supplied to the closed container 110 from the supply vent 120. The gasified adhesive can then adhere and be coated on the SiO2 layer. The operation time of this step is approximately forty seconds. Next, in step 250, the gas is exhausted from the first exhaust pipe 140 and ceases to be exhausted from the second exhaust pipe 150, and the remaining gasified adhesive in the closed container 110 is pumped out. The operation time of this step is approximately seven seconds. Finally, in step 260, the coating process of applying the adhesive on the upper oxidation layer of the wafer 100 is accomplished. The method as mentioned above can be applied to such machines as the TEL-MK5Z (produced by Tokyo Electron Limited of Japan) and the like.
In order to coat the photoresist precisely and uniformly on the SiO2 layer, a method of spin-coating is generally used. In spin-coating, a spinning apparatus drives the wafer 100 to rotate. The photoresist is dropped on the SiO2 layer and moves toward to the edge of the SiO2 layer due to the centrifugal force from spinning. In this way, a precisely and uniformly thick layer of the photoresist is eventually formed on the SiO2 layer.
An objective of applying an adhesive as mentioned above is to have a SiO2 layer coated with a photoresist in the following micro-fabrication, ensuring the photoresist adheres to the SiO2 layer tightly. However, due to the influence of the centrifugal force resulting from the rotation of a spinner during the photoresist application process, the binding force of the adhesive is often insufficient, resulting in the photoresist instability and subsequent flaking off the SiO2 layer. This is the so-called resist lifting issue. Therefore, the conventional applying method for an adhesive results in the photoresist degradation and flaking off the SiO2 layer very easily.